Preparation And Structure-activity Relationship Of Noble Metal Catalysts In Confined Nano-space/Matrix

Supported metallic nanoparticles catalysts have extensive applications in the field of fine chemicals hydrogenation and fuel cell. The activity and stability of the supported metal catalysts are two essential scientific issues in catalysis. With the development of nanoporous material, the confinements of nanopore effects on the nanoparticles becomes a hot topic in the field of catalysis and materials. The electron configuration of metal nanoparticles can be modified due to the nanoscale spatial confinement, which could directly influence their catalytic activity through interaction with reactants, intermediates and/or products. Herein, the effects of the confinements of the nanospace on the catalytic performance and stability of the nano Pd and Ru metal catalysts has been investigated. The main results are the followings:1) The yolk-shell nanospheres with soluble amino-polystyrene as core material(PS-NH2@meso Si O2 YSNs) have been synthesized by nitration and reduction of polystyrene nanospheres(PS) confined in silica hollow shell. The catalytic performance of Pd/PS-NH2@meso Si O2 was investigated via the selective hydrogenation of acetophenone(AP) to produce a-phenyl ethanol(PE). The amino-polystyrene(PS-NH2) confined inside mesoporous silica shell can be used as an efficient and robust solid stabilizing reagents for Pd NPs. High content of NH2 groups in the core of PS-NH2@meso Si O2 yolk-shell nanospheres not only results in Pd NPs with small particle size but also provides basic surroundings for suppressing the hydrogenolytic splitting of the C-OH to improve the selectivity for α-phenyl ethanol. Both the thermal stability and anti-swelling ability of PS-NH2 in the core of yolk-shell nanospheres are greatly enhanced due to the confinement effect of mesoporous silica shell. The 1wt%Pd/PS-NH2@meso Si O2-L could be stably reused for more than 10 times without obvious loss of activity and selectivity.2) A yolk-shell nanoreactor with encapsulated Ru-PVP nanowires in ethane–silica hollow nanospheres(Ru-PVP@Si O2 YSNs) for low temperature F-T synthesis was prepared with Ru-PVP nanowires, tetraethoxysilane and 1,2-bis(trimethoxysilyl)ethane via a organosilane-assisted selective etching strategy. Under similar reaction conditions(3.0 MPa, 150 oC), the Ru-PVP@Si O2 YSNs shows higher activity(activity: 6.35 versus 5.96 mol COmol-1Ruh-1) and selectivity towards oxygenate products(41.3 versus 21.6%) than free Ru-PVP nanowires in aqueous F-T synthesis. Encapsulation not only give rise to the quasi-homogeneous Ru-PVP with facile recycling ability, but also enhanced its activity and selectivity towards oxygenates. The high activity and selectivity of the encapsulated Ru-PVP is mainly attributed to the low PVP/Ru ratio in increasing the degree of exposure of the active sites and the unique yolk-shell nanostructure in increasing the secondary reaction and re-adsorption of intermediate such as α-olefins. Through the combination of highly active quansi-homogeneous Ru-PVP with ethane-silica hollow nanoreactor, the solid catalyst could be easily separated from the reaction system by filtration. The activity remains almost the same and varies in the range of 5.52-5.98 mol COmol-1Ruh-1 during 4 times recycle processes.3) Ruthenium-containing ordered mesoporous carbon(Ru-OMC) catalysts with highly dispersed Ru nanoparticles semi-embedded in carbon framework were prepared via a direct Ru Cl3/SBA-15 hard template method. Furthermore, an in-situ IR spectroscopic combined with thermo gravimetric technique was applied in the carbonization process of sucrose-Ru Cl3/SBA-15 composite towards a Ru-OMC catalyst to identify the stabilization role of sucrose during the formation of highly dispersed Ru nanoparticles. The results show that the formation of metal carbonyl species results in a formation of homogeneous distributed Ru nanoparticles, and the rigid silica support and carbon framework around the Ru(CO)x complex can significantly avoid the sintering and agglomeration of Ru metal particles during elevated temperature thermal treatment. The Ru-OMC catalyst exhibit more excellent activity of in aqueous-phase F-T synthesis compared with some other carbon supported Ru catalysts(3.0 MPa, 150 oC, activity: 15.7 versus 6.9 mol COmol-1Ruh-1) which due to the embedding of the Ru NPs. The intimate contact between the embedded Ru nanoparticles and the carbon support which facilitates the hydrogen spillover and thus improves the hydrogen disassociation on the catalyst surface.4) Ru-OMC with various embedding degrees has been fabricated by using a boric acid-assisted hard template method. The relationship between embedding degrees of Ru nanoparticles and catalytic performance was determined by using toluene hydrogenation as a model reaction. To study the effect of pore size on the catalytic performance, a series of OMC supported Ru catalysts(Ru/OMC) with various pore sizes were also prepared and compared with Ru-OMC catalysts. At 110 °C and 4.0 MPa, the catalysis toluene hydrogenation activities of embedding Ru-OMC catalysts are much higher than those of supported Ru/OMC catalysts which can be attributed to the strong interaction between ruthenium nanoparticles and the carbon support. Furthermore, the activities of Ru-OMC catalysts are closely related to the embedding degree of ruthenium nanoparticles in the carbon support, which appears as a volcanic type curve. The Ru-OMC catalyst with an embedding degree of 12.6 % affords a turnover frequency of up to 4.69 s-1 in toluene hydrogenation.In summary, confined metal nanoparticles inside the hollow silica shell not only maintain the activity but also improve the separation and recycle stability. Meanwhile, embedded metal nanoparticles in carbon matrix could significantly avoid the sintering of noble metal particles, and enhance the activity of hydrogenation. Therefore, confinement of metal nanoparticles is a great alternative to solve the problem of catalytic activity and stability of supported noble metal catalysts.